How come these genrates different opcodes?
push dword 0
push 0

I want the short version when possible, but if I don't put dword after all of my pushes then fasm will error out on indirect memory references. Making my compiler check for this and have it sometimes generate "dword" and sometimes not when it always wants to push a dword seems a bit unneccesary. Can I tell fasm to prefer the short version?

Yes, use the PUSHD mnemonic - it defaults to 32-bit while optimizes imm pushes just like the plain PUSH.

If I do

it doesn't work, but with push dword [v_s] it works.

That's because you can't push a byte onto a dword stack; because you defined it as "rb 4". Change it to "rd 1" or "dd ?" and it should work. See the FASM principles (etc blah blah blah) in the Documentation section, Tomasz will surely link it

The reasoning for this is that tries to push [v_s] straight onto a dword stack - which can't be done is overriding the byte assignment into a dword, which FASM allows to be pushed onto a dword stack.

Now, remember that is always available for less confusion - but it might also make it more confusing

Hope you understood that.

P.S. Please correct me if I'm wrong. I am at a friend's house, so no FASM to test